Recent investigations into the causes of death from asthma in the community have suggested that major problems exist in the management of this common disorder. Although several factors have been identified that may contribute to death, the single most important one is a failure, not only by the patient and relative but also by the doctor, to assess and appreciate the severity and speed of onset of an acute attack, resulting in delay in initiating appropriate treatment. An explanation that could account for this observation is that symptoms alone are commonly used to assess the severity of an asthma attack, despite studies having established this approach as inadequate (Burdon J. G. W, et al, "The Perception of Breathlessness in Asthma", Am Rev Respir Dis, 126:825-828 (1982)). The studies found that some asthmatic patients may be symptomless in the presence of substantial air flow limitation and that patients with the most reactive airways commonly exhibit minimal symptoms during an asthma attack.
The above findings suggest that regular assessment of airflow obstruction by objective means is necessary for appropriate management of these patients. One such objective assessment is a measurement of peak expiratory flow (ie. the maximum flow rate of a single forced expiration).
A variety of mechanical devices for measuring the maximum flow rate of a single forced expiration have been proposed to date. By way of example reference should be made to New Zealand Patent Specification No. 172981 (Allen & Hanburys) and to British Patent Specification Nos 1344836 and 1351112 (Ferraris Development) which describe such devices. Further and particular reference should also be made to British Patent Specification No. 1463814 and U.S. Pat. No. 3,958,565 which describe the ventilatory capacity measuring instrument developed by B. M. Wright, conventionally known as the Wright peak low meter.
However, most if not all of these prior art devices suffer from the disadvantages that they are less than accurate over the full range of expiratory flow rates. Moreover, such devices tend to be bulky and relatively large in size. In consequence, they are not generally carried by asthmatics.
It is an object of the present invention to provide a device for measuring peak expiratory flow which goes some way towards overcoming the above disadvantages or which at least provides the public with a useful choice.